Modern aircraft include generators that generate power during flight and provide the generated power to onboard aircraft electric power systems. The generators utilize rotation of the aircraft engine to generate AC power using known power generation techniques. Power generated in this manner is typically 230V 400 Hz AC power. While the aircraft is on ground, aircraft engines can be turned off, the onboard generator ceases generating power, and the onboard electric system instead receives AC power from a ground cart. Power provided from the ground cart is typically 115V 400 Hz AC power.
While the power sources provide AC power, aircraft components often require DC power instead of AC power. AC-DC power conversion may be accomplished with a plurality of diode pairs, where each pair is connected to a different phase of the AC input, to provide a rectified DC output. However, this type of AC-DC conversion leads to substantial current harmonics that pollute the electric power generation and distribution system. To reduce current harmonics, multi-phase autotransformers are employed to increase the number of AC phases supplied to the rectifier unit. For example, in an 18-pulse passive AC-DC converter the autotransformer is used to transform the three-phase AC input, whose phases are spaced at 120°, into a system with nine phases spaced at 40°. This has the effect of reducing the harmonics associated with the AC-DC conversion.